Rotary offset printing presses reproduce an image on a substrate comprising successive sheets of paper or a web of paper by means of a plate cylinder which carries the image, a blanket cylinder which has an ink transfer surface for receiving the inked image, and an impression cylinder which presses the paper against the blanket cylinder so that the inked image is transferred to the paper. Lithographic inks applied to the paper can be partly absorbed and dry mainly by oxidation. Such inks are strong relative to other inks, do not contain aqueous solvents and generally have a very high solids content. Drying of lithographic inks can be enhanced by oxidation at somewhat elevated temperatures.
Many modern presses employ a coating or “lacquer” unit at the end of the press which can employ flexographic, ultraviolet (UV) or aqueous based coatings or printing inks quite different from lithographic printing ink. One example is a coater/printer made by Printing Research, Inc. illustrated in U.S. Pat. No. 5,176,077. In addition, coating equipment has been made for use with one of the regular lithographic printing stations on a printing press. These include retractable interstation coating units which permit flexographic, UV or aqueous based coatings and/or printing to be done at any desired station on a printing press in addition to the last station. Examples of such coating equipment produced by Printing Research, Inc. are illustrated in my U.S. Pat. Nos. 5,960,713, 5,651,316 and 5,598,777.
When conveyed through a printing press, freshly printed sheets are delivered to a stacker where they are collected and stacked. The wet ink and coating should be dried before the sheets are stacked to prevent smearing defects and to prevent offsetting and “gas ghosting” of the ink on the unprinted or printed side of the sheets which may occur when one sheet is stacked on the next sheet. Spray powder is usually applied to freshly printed sheets to be stacked for the purpose of preventing offsetting of freshly printed sheets. The use of spray powder is not desirable for other reasons. It can cause a rough feel to the printed surfaces of sheets and builds up on plates and blankets where it can interfere with good printing quality. This causes more frequent shutdowns to wash plates and blankets and is also detrimental to press components. The present invention reduces or eliminates the need for spray powder. Although spray powder can prevent offsetting while the ink and/or coating dries, this is only a partial solution to drying problems at best. In the case of flexographic, UV and/or aqueous based coatings or printed images which have relatively heavier wet film thicknesses, auxiliary drying before stacking is a necessity because of the difficulty of drying heavy wet ink films, especially aqueous based inks or coatings.
Hot air convection heaters and radiant heaters have been employed in dryers after printing and coating stations on printing presses. These are best suited for slow to moderate speed press runs in which exposure time of each printed sheet to the hot air convection flow is long enough that aqueous based inks and coatings are set before the sheets reach the stacker.
For high speed press operation, for example, at 5,000 sheets per hour or more, good drying is not generally obtained by convection flow alone. Improved dryers have been produced which employ infra-red heat lamps to provide greater drying efficiency because the short wave length infra-red energy is preferentially absorbed in the liquid inks and coatings to promote rapid drying. Infra-red radiant energy releases water and volatiles from the inks and/or coating. Scrubbing the printed surface with high velocity air further promotes drying. An example of a dryer that functions using a combination of high energy infra-red heat lamps together with high velocity air and extraction of the spent volatiles and water vapor is found in an infra-red dryer described in my U.S. Pat. No. 5,537,925 sold by Printing Research, Inc., which is incorporated herein by reference. This equipment in modified form is utilized in the present invention.
One of the problems with some prior art infra-red (IR) dryers is the fact that they must extend the fill width of the substrate width capacity of the press and they generally operate by off-on control. All of the heating tubes in the dryer are turned on when the press is printing and turned off when the press is stopped. If the press is printing a job where the substrate is less than the full width of the printing press, lamps in the IR dryer are being powered in areas where no substrate is being heated under them. This is no small matter, because powerful IR lamps are being employed to accommodate faster press speeds. In the incorporated U.S. Pat. No. 5,537,925, the lamps were each 1 kw lamps. In the preferred embodiment of the present invention, the lamp power consumption has been increased to 2 kw and there may be as many as 33 or more of these lamps in a dryer head. If, for example, a 24 inch sheet is run through a 40 inch press with such a dryer, 8 inches on each side does not need to be heated because there is no substrate there and no ink to dry. This kind of prior art dryer will continue to apply power across the full 40 inch (102 cm) width with a corresponding waste of expensive electricity and generation of unnecessary heat in the press and the pressroom.
One prior art dryer is an improvement to the typical all lamps on or all lamps off configuration of most prior art printing press dryers. It is known as the Air Blanket Infra Red Dryer sold by Printing Research Inc. which is the commercial embodiment of the dryer disclosed in my U.S. Pat. No. 5,537,925. The outer lamps are wired in groups of two, but the centrally located lamps are connected to operate as a single group of lamps. There may be two or three of the outer groups of lamps which operate in pairs. For example, the two left side outermost lamps and the two right side outermost lamps can be turned on or off together. The next two pairs of lamps on the left and right can be turned on and off together. There may be a third group of paired lamps. These paired lamps (two on each side) are connected to a selector switch which enables the operator to turn off two lamps on each side, four lamps on each side or six lamps on each side. This helps to save energy but the main group of lamps in the center is not affected and still operate together as one large group subject only to off-on control. Importantly, none of the groups of lamps in this prior art design, nor any individual lamp, is independently controlled in response to the temperature of the sheet. Power to the prior art dryer mentioned above is fixed by a selector switch and/or rheostat device and must be set initially and reset manually if the operator perceives that printed sheets are coming off too hot or too cold.
In addition, it is known that areas containing only text may require little or no drying whereas areas containing heavy coverage need considerably more drying power. There also may be non-printed areas which are devoid of any printing, have very little printing or have a kind of printing which does not require drying at all. One example of this may be “work and turn” jobs where one half of the sheet has process colors and the other half has text. After printing the first side, the sheets are turned over and run back through the press where the printing is repeated on the other side. The area which has only text, requires very little drying and with prior art dryers, has been subjected to high intensity radiation twice. Another example is the use of IR dryers on two color presses where four color jobs have to be run through the press two or more times. Areas not having ink are subjected to intense IR energy which may remove too much moisture and dry out the sheets. This can also affect register if one part of the sheet has more moisture than another part. Although the cost of energy is high in this country, there are a number of foreign countries where electrical energy costs three to four times as much as it does here. The energy savings is significant.
It is also a desirable goal to try to maintain the substrate temperature at a slightly elevated but uniform temperature across the surface measured at different points across the width and down the length of the substrate sheets. Powerful infra-red energy is applied from lamps operating at 120 to 480 volts. Despite attempts to moderate the effect of such intense radiation, temperature variation in the sheet continues to be a problem which is exacerbated when the sheets are stacked such that heat cannot readily escape and heat build up in the stack can occur. Some heat build up in a stack occurs naturally as a result of the oxidation process in lithographic inks. Nonuniform temperature can affect moisture content and a tendency for curling of the sheets. High temperature areas can increase the tendency for offsetting and sticking/blocking of sheets. This dryer helps prevent blocking. Temperature non-uniformity is believed to occur because the printed sheet has varying amounts of ink with different colors in different areas which absorb more or less infra-red radiant energy. Areas which are mostly white may not absorb as much of the infra-red energy with a resulting lower temperature in that area of the sheet. On the other hand, heavily printed areas with a dark color such as black, may readily absorb greater quantities of infra-red heat energy thus raising the temperature of the sheet nonuniformly. The present invention is directed to the reduction of energy cost and solution of these printing problems.